The present invention relates to bushings and, more particularly, to a composite and self-lubricating bushing for use in die sets, presses and other heavy duty machinery. The invention also relates to a method for the manufacture of a cylindrical bearing surface finding advantageous use in such a bushing.
It is well known in the tool and die industry that longlasting, precision bushings are an important component in the design of commercially acceptable die sets. Because these bushings are often subjected to high press velocities and substantial side thrust forces, it is necessary that they be formed from a monolithic block and that they be carefully constructed to exacting specifications. Two types of plain guide bushings are well known to the art and normally available from most die set manufacturers: hardened steel bushings and plated bushings in which a thin layer of bronze is plated in the bore of a steel bushing. In either case, the bushing is closely fitted to a hardened and ground guide post with a diametrical clearance ranging from about 0.0003 to 0.0008 inches. Blanking and piercing dies must be closely fitted to avoid shearing the cutting edges, while forming dies will work well with larger clearances.
Hardened steel bushings are widely used at present and, when properly lubricated and maintained in alignment, will provide excellent service and wear life at moderate press speeds. For applications with higher speeds or higher side loads, bronze plated bushings are preferred because they reduce chances of galling or seizing. These bronze plated bushings must also be lubricated regularly to avoid failure.
Some prior art dies, where greater pin and bushing clearance can be tolerated, may be fitted with solid bronze bushings inserted into the steel or iron body of the die. These bushings are more tolerant of dirt or fine metal particles which find their way into the space between the guide post and bushing and become embedded in the bronze. A plated bushing would have too thin a layer of bronze to accept anything but the finest of foreign particles and the hardened steel bushing would be even less tolerant. The disadvantages of solid bronze are lower mechanical strength, greater expense and a coefficient of expansion significantly greater than that of steel. Thus, as the bushing heats in service it tends to close in on the guide post adding to the heating problem. Sufficient clearance and lubrication must be provided to avoid this problem.
It is well known to provide all of these prior art bushings with means for lubrication, such as a lubricating fitting so that grease or other lubricants may be periodically introduced to the internal bearing surface. However, under high velocity and extreme load conditions such lubricants are quickly dissipated; and if the tool operator is not diligent in the proper and periodic application of lubricant, it is possible that a bushing may seize despite all of the foregoing design precautions. Thus, a need exists in the industry for a quality bushing for use in die sets and in other high load applications which is capable of self-lubrication for extended periods of service.
One of the important properties of bronze as a bearing material is its ability to conduct heat away from the bearing surface. For example, coefficients of heat transmission for bronze are about five times greater than steel. Therefore, a need exists for a bushing having a lining of bronze thick enough to conduct heat away but not so thick as to cause substantial reduction of the clearance between guide post and bushing, which can occur due to the fact that the heat expansion of bronze is 57% greater than that of steel.
Another well known bearing material is a porous sintered bronze which has the advantage of holding a lubricant in as much as 25% of its volume. However, most of the known prior art applications of such a sintered bronze have involved bearings fabricated from sheet stock. When formed into a bearing, the body of such bearings includes a separation line typically formed when the sheet stock is rolled into a cylindrical configuration. Thus, the resulting bushing is not a monolithic structure. Accordingly, these bearings are used only in commercial applications under conditions of moderate loads. They are not used as guiding elements in die sets, presses or other heavy machinery primarily because of the difficulty of achieving close, accurate fits.
It is also known in the prior art to introduce a sinterable particulate material into a bushing's annular cavity, and compact the particulate material against the inner surface of the bushing in situ by means of a rigid and tubular-shaped compacting punch which enters the space between a center plug and the interior die wall. This compaction is accomplished by insertion of the compacting punch into each end of the bushing. Such "end compaction" achieves poor results because the center portion of the porous bearing layer may be improperly compacted due to side wall friction between the particulate material and the annular cavity between the center plug and the interior die wall. Porous bearing layers in this state simply cannot be properly sintered and machined to a dimensionally uniform state, which state is required for proper die set use, and other similar applications. This problem is exacerbated in thin porous bearing layers used with bushings of substantial length, such as those used with die sets. For example, the inventor has experimentally verified that functional losses in a sleeve bushing compact having a wall thickness of 0.030 inches and a length of 3.00 inches are so great that 98% of the compacting pressure is lost at the center of the bushing, even when pressing from both ends. With such slight pressure applied at the center of the bushing, the integrity of the compacted layer is insufficient to maintain the physical structure of that porous bearing layer necessary for sintering purposes. Thus, the thin-walled porous bearing layers necessary for use in die set bushings cannot be obtained with known prior art.